Tire engineers have historically attempted to build very durable casing structures that can survive the severe driving conditions that the vehicle operators put the tires through.
Early on tires were very heavy and employed many layers or plies of bias cord. The primary objective was to simply retain the air and avoid a flat or deflation.
Through a process of endless research to develop more durable and better tire constructions new materials and better designs have been developed.
The introduction of the radial tire made it practical to develop tires having as few as one carcass ply. The ply was contained radially by a belt structure. To enhance the durability of the tire these belt structures evolved to become primarily steel reinforced. These steel reinforced belts yielded and currently provide a very durable structure.
These steel belted tires have many benefits that make their use attractive. The steel cords are not heat sensitive that is their physical properties are pretty much constant regardless of the tire's operating temperature. The steel cords are substantially inextensible and the cords can be made very high strength with fine filaments that have excellent fatigue resistance. Nevertheless these steel corded belts in tires have resulted in the need to add rubber gauge directly above the belts in the area commonly referred to as the undertread, in the belt layers themselves and in the areas of the belt edges, all in an attempt to keep these steel cords from becoming exposed or structurally separating at the edges. Furthermore, the glass reinforced cord has a tenacity of 10 gr/denier compared to steel cord used in belts having 4 gr/denier.
The resultant effect has been that the steel belted radial tires are in fact heavier using more rubber in the tread area and in the tire's shoulders. It is precisely in these areas that a large part of the tire's tread wear performance and sensitivity to rolling resistance must be the highest. The more rubber in this area the higher the hysertesis effects and higher the temperatures under running conditions.
It is now an object of tire designers to develop tires that generate lower car fuel consumption. This can be achieved by designing cool running tires that have low mass and low rotational inertia while increasing the tire's handling performance and treadwear, furthermore, the engineer must insure that the tire's footprint and the contact patch of the tread has a uniform pressure distribution in order to achieve uniform wear.
With the advent of high performance tires having very low aspect ratios the use of belt structures having overlays of synthetic cords of nylon or aramid has been common. To further achieve high speed performance the tread thickness has been kept at a minimum. Thick tread mass at high speeds simply want to fly off the tire. As these tires are pushed to the engineer's known tire design limit he must rethink the entire parameters of the tire. In some cases this means going back and reanalyzing the concepts that were used in the past but were abandoned as a result of those in the art pursuing a different path.
One such approach that heretofore had lost favor among tire engineers was the use of fiberglass belts which although a very good material for belts lost favor when steel belts were introduced. The primary downfall of fiberglass belts was their apparent lack of durability.
Even in the bias tires of the 1970's the use of fiberglass breakers had technical concerns. In U.S. Pat. No. 3,762,458 issued Oct. 2, 1973, to Yoshida, et al., stated that "Glass is superior to organic fiber cord in the heat resistance, dimensional stability and modulus of elasticity and when rubber is reinforced with glass cord and used as a breaker layer of a pneumatic tire, the tire is excellent in various properties, particularly is excellent in the abrasion resistance (road test) and cornering power."
Yoshida et al., then goes into detail as to why fiberglass cords were being disfavored in their use as breaker cords. He cites three serious drawbacks of the use of fiberglass breakers.
"Firstly, when a car is running, dynamic bending transformation and impact transformation of the tire occurred due to the road surface condition, and the glass cords are broken or crushed.
Secondly, in general, foreign materials, such as nails, glass pieces and gravels, are penetrated into the tire and reached its breaker layer during the use of the tire, particularly at the end of the use. In this case, if the tire has a breaker layer composed of conventional organic fibers, such as nylon, rayon, polyester and vinylon (polyvinyl alcohol) fibers, etc., breakage of the tire occurs only at the portion to which the foreign materials have been penetrated. On the contrary, if the tire has a glass cord breaker layer, the glass cords are crushed by the penetrated foreign materials and the breakage of the glass cords extends along the glass cord breaker layer. This is a cause of serious troubles.
In order to solve these drawbacks, there has been tried to arranged organic fiber cords, such as nylon cords, on he tread side of the glass cord breaker layer. However, a satisfactory result has not hitherto been obtained.
In addition to these drawbacks of glass cord, the inventors further have found a third drawback of glass cord, which is peculiar to glass cord and does not occur in organic fiber cord.
That is, in the vulcanization step of tire, the following step may be carried out. Namely, a tire is vulcanized at a high temperature and under a high pressure, and then the tire is taken out into an atmosphere at room temperature and under atmospheric pressure, and thereafter the tire is applied with air pressure to the inner side under a high pressure to stabilize the dimension of the tire. When the tire is taken out into the atmosphere at room temperature, cords composed of organic fibers other than glass fiber, such as rayon, nylon, vinylon and polyester fibers, etc., which are used as a reinforcing material of tire, that is, used in a carcass arranged on the inner side of breaker, are highly shrunk. As the result, glass cord breaker arranged directly adjacent to the rubber coated organic fiber cord layer (carcass) is compressed violently, and glass filaments constituting the glass cord of the breaker are compressed to decrease their tenacity and are broken during the car is running.
For instance, in a tire composed of 2 kinds of layers of a carcass layer reinforced with organic fiber cord and a glass cord breaker, glass cords constituting a glass cord breaker layer arranged on the carcass side are inferior in the tenacity to glass cords constituting another glass cord breaker layer arranged on the tread side.
Further, when organic fiber cords having different shrinkabilities, for example, nylon cord or rayon cord is used as a carcass cord, the nylon cord having a higher shrinkability decreases the tenacity of glass cord more than the rayon cord.
Further, even in the vulcanization treatment of conventional bias-belted tires in which rubber coated organic fiber cord layer, such as rubber coated nylon cord layer, is arranged on the tread side of the glass cord breaker, the above described phenomenon occurs between the glass cord breaker and the rubber coated organic fiber cord layer, whereby glass filaments are broken. Because, when the tire is cooled, the above described organic fiber cord shrinks considerably."
These three problems were allegedly solved by Yhoshida et al., as quoted below:
"Among the above described three drawbacks of the tire having the glass cord breaker layer, the first drawback has already been solved by the inventors. That is, in order to decrease the bending transformation and impact transformation of glass cord, a short cut fiber reinforced rubber layer is arranged on the tread side of the glass cord breaker layer. In order to solve the second drawback, that is, the problem of plunger resistance against foreign materials, the inventors have confirmed that the arrangement of the short cut fiber reinforced rubber layer on the tread side of the glass cord breaker layer has a higher effect than the arrangement of a sole rubber layer or a rubber coated cord layer.
Furthermore, in order to solve the third drawback, the inventors have found that the following arrangement is more effective. That is, the glass cord breaker layer is not arranged directly adjacent to organic fiber cords having a high shrinkability, but such a layer that has a low shrinkability and does not transmit the shrinkage of the organic fiber cords to the glass cord breaker layer is arranged between the rubber coated organic fiber cord layer (carcass) and the glass cord breaker layer."
Thus, Yoshida et al., attempted to retain the use of fiberglass breakers in bias tires.
The use of fiberglass belts in radial tires was even more challenging. In U.S. patent to Christian M. L. L. Bourcier de Carbon of France, de Carbon explains "Radial carcass tires essentially comprise a radial carcass, made up of curved members which are straight in the center planes of the cover, in combination with a non-extensible belt comprising a reinforcement which is flexible in the radial direction (the direction perpendicular to the tread surface) but has high longitudinal and transverse rigidity (i.e., in directions parallel to the tread surface), the non-extensible belt being automatically tensioned by the internal pressure and thus having a considerable equatorial binding effect on the curved members of the carcass, even when the pneumatic tire is at rest and is not subjected to any crushing load."
DeCarbon's solution to the problem of radial tire belt structures was the use of flattened cords having a width of about 1 mm, the highly flattened cords being of steel plastic or glass fiber oriented at cross angles of about 45.degree. to 60.degree.. As can easily be appreciated such solutions were very complicated and ultimately lost any commercial interest due to their complexity and the concurrent simpler success of steel belts.
The present invention demonstrates that the use of fiberglass belts can again be made commercially acceptable if the fiberglass is employed with a combination of other components that insure that the glass cords are not damaged during manufacture by insuring that the thermal shrinkage differentials created during vulcanization, cooling and subsequent reinflation are not transmitted to the glass cords. The fiberglass belts not only are commercially acceptable but also can provide surprisingly beneficial improvements in tire weight and reduced rolling resistance.